Large, lightweight mirror blanks are commonly produced from materials having low coefficients of thermal expansion (CTEs), particularly over the temperature range of -50.degree. to +100.degree. C. This minimizes the effect of temperature changes in the ambient atmosphere when measurements are made with a telescope.
Materials presently used include fused silicas and glass-ceramics having CTE values in the range of .+-.10.times.10.sup.-7 /.degree. C. over the 0.degree.-300.degree. C. range. The fused silicas currently employed are either an essentially pure silica or a silica doped with about 7.5% TiO.sub.2. The latter is described in detail in U.S. Pat. No. 2,326,059 (Nordberg), and is hereafter identified as a TiO.sub.2 -doped, fused silica glass.
The fused silicas are customarily produced by a chemical vapor deposition process. In this process, oxide precursors, usually metal chlorides in vapor form, are introduced through a burner flame to produce molten oxide particles. These molten particles are deposited on a large support member to build up a body termed a boule.
The production process for a fused silica-type mirror blank involves the following steps:
1. Forming fused silica boules. PA0 2. Slicing the boules to appropriate size. PA0 3. Cutting the boules into hex shapes. PA0 4. Fusing the hexes together at a high temperature to form a unitary blank. PA0 5. Grinding flat, top and bottom surfaces (plano surfaces) on the blank. PA0 6. Grinding the edge of the blank to an approximate outer diameter for the mirror. PA0 7. Sagging the plano blank over a mandrel to a desired curvature. PA0 8. Grinding the convex and concave surfaces of the blank to final contour. PA0 9. Grinding the edge of the blank to a final outer diameter.
Large diameter blanks normally have high diameter to thickness ratios. Consequently, the blanks must be well supported from underneath to prevent deflection during grinding. Inadequate support during grinding allows the blank to deflect under its own weight and by the forces generated by the grinding wheel. Deflection during final grinding will result in mechanical stress in the blank, out of tolerance surface profiles, or a combination of both. If the deflection is high enough, the resulting stress may cause the blank to break on the grinder.
For a given support condition, the total stress at any location in the blank is the resultant of the internal and external forces acting at that location. Internal forces are due to effects such as CTE variations, and would be present even in a zero gravity condition. External forces are those which act on the blank from outside, such as gravity, support reactions and grinding. External stresses are also referred to as mechanical stresses.
Mechanical stress in the blank can increase the amount of material that the finisher must remove from the blank. Also, there is generally a stress limit specified for the blank. It is conceivable then that the blank could be rejected, or require extensive rework, if the combined internal and mechanical stress in the blank exceeded this limit.
In the past, lightweight blanks have been supported on blocking bodies during grinding. These bodies have been made from the same material as the mirror. A blocking body is a rigid structure which is used to support the blank from underneath, thereby limiting deflection in the blank. Typically, three blocking bodies are required in producing a convex/concave blank with ground surfaces. These blocking bodies are characterized by the shape of the top surface which supports the blank (plano, convex, and concave).
Previously, carpeting and foam rubber have been used as compliant, interface materials between the blocking body and the blank. This compensates for contour mismatches between the blank and blocking body contact surfaces. The compliant material acts as a series of springs, spreading out the support over a large area. This eliminates point loading in the blank.
The mirror material has proven to be an excellent blocking body material due to its mechanical and thermal stability. However, it is extremely expensive. As the blanks continue to increase in size, cost and capacity considerations preclude using such materials as a blocking body material.
Consequently, an effort has been made to use an alternate material, referred to in the trade as polymer concrete, for blocking bodies. This is a mixture of granite, or similar material, with 5-10% of a polymeric material as a binder. While a polymer concrete body is much less expensive than a fused silica glass, it also lacks the thermal and mechanical stability of the glass.
Another problem arises because neither the fused mirror blank, nor the blocking body, has a perfectly contoured mating surface. As a result, when the blank is placed on the blocking body, there are areas where the mating surfaces do not meet. When the former carpet or foam, spring-type, interface materials are used for support, deflection of the glass, and consequent mechanical stress, result in the workpiece during grinding. Contour mismatches then are a problem for two reasons. One is the temperature and mechanical instability of the blocking body. The other is the inability to sag blanks to exact contours.
It is then a basic purpose of the present invention to provide a novel and improved method of supporting large mirror blanks during grinding. Another purpose is to provide a method that minimizes deflection-related stresses being induced during grinding. A further purpose is to provide a method that permits use of polymer concrete, or a similar low cost material, for blocking bodies.